I don't think this song is about hiding your true self, I think it's about a woman who is influenced by the media and society. Her definition of beauty is what advertisers have made it up to be. Make-up covered plastic.
To me this song isn't from the woman's point of view, but from an outsider's who is almost mocking her way of thinking. "Am I made of plastic? Am I made of sand?" She fills her body with these unnatural things to the point where she isn't even flesh and bones even more(obviously not literally). She's so fake and plastic that she would have to question what she's made out of if she ever thought about it. "Am I beautiful or worthless? Like there ever was a difference" is such a powerful line. It's saying that looking like the mechanical supermodels we're pressured to admire doesn't make you anything more than what you are. Being "beautiful" isn't everything in life, though some want us to believe that. Because after all, what IS "beautiful" and why do some people feel they have the right to answer that for us?